Pharmacological activators of
peroxisome proliferator-activated receptor-gamma (
PPARgamma) inhibit growth of
non-small-cell lung cancer (NSCLC) cell lines in vitro and in xenograft models. Because these agents engage off-target pathways, we have assessed the effects of
PPARgamma by overexpressing the
protein in NSCLC cells. We reported previously that increased
PPARgamma inhibits transformed growth and invasiveness and promotes epithelial differentiation in a panel of NSCLC expressing oncogenic K-Ras. These cells express high levels of
cyclooxygenase-2 (COX-2) and produce high levels of
prostaglandin E(2) (
PGE(2)). The goal of these studies was to identify the molecular mechanisms whereby
PPARgamma inhibits
tumorigenesis. Increased
PPARgamma inhibited expression of COX-2
protein and promoter activity, resulting in decreased
PGE(2) production. Suppression of COX-2 was mediated through increased activity of the
tumor suppressor
phosphatase and
tensin homolog, leading to decreased levels of phospho-Akt and inhibition of
nuclear factor-kappaB activity. Pharmacological inhibition of
PGE(2) production mimicked the effects of
PPARgamma on epithelial differentiation in three-dimensional culture, and exogenous
PGE(2) reversed the effects of increased
PPARgamma activity. Transgenic mice overexpressing
PPARgamma under the control of the
surfactant protein C promoter had reduced expression of COX-2 in type II cells and were protected against developing lung
tumors in a chemical
carcinogenesis model. These data indicate that high levels of
PGE(2) as a result of elevated COX-2 expression are critical for promoting lung
tumorigenesis and that the antitumorigenic effects of
PPARgamma are mediated in part through blocking this pathway.